A wide variety of implantable medical devices for delivering a therapy or monitoring a physiologic condition have been clinically implanted or proposed for clinical implantation in patients. For example, implantable medical devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic stimulation to the heart by delivering electrical therapy signals, such as pulses for pacing, or shocks for cardioversion or defibrillation. In some cases, such an implantable medical device (IMD) may sense for intrinsic depolarizations of the heart, and control the delivery of such signals to the heart based on the sensing. When an abnormal rhythm is detected, which may be bradycardia, tachycardia or fibrillation, an appropriate electrical signal or signals may be delivered to restore the normal rhythm. For example, in some cases, an implantable medical device may deliver pacing, cardioversion or defibrillation signals to the heart of the patient upon detecting ventricular tachycardia, and deliver defibrillation electrical signals to a patient's heart upon detecting ventricular fibrillation. Pacing signals typically have a lower energy than the cardioversion or defibrillation signals.
In some cases, an IMD device includes sensors for detecting heart sounds signals in addition to electrical signals. As described in Published U.S. Patent Application 2010/0331903 to Zhang et al. entitled “HEART SOUND SENSING TO REDUCE INAPPROPRIATE TACHYARRHYTHMIA THERAPY”, incorporated herein by reference in its entirety, heart sounds signals are detected and used to determine whether the heart sounds are normal or abnormal. The heart sounds may also be used to confirm or reject an indication that therapy may be needed based on electrical signals.
Heart failure is a condition affecting thousands of people worldwide. Essentially, congestive heart failure occurs when the heart is unable to pump blood at an adequate rate in response to the filling pressure. A worsening heart failure condition may result in symptoms such as congestion in the tissue, peripheral edema, pulmonary edema, and shortness of breath, and coughing. When heart failure is severe, it can even lead to patient death.